1. Field of the Invention
This invention relates to the field of dry etching of aluminum and more specifically to the reactive ion etching of an aluminum layer in the fabrication of a semiconductor device.
2. Prior Art
Aluminum or aluminum alloyed with small amounts of silicon or copper, or both, are most commonly used in the interconnection metallurgy of integrated circuits. As modern semiconductor devices decrease in size to meet the requirements of high speed, high packing densities, the widths of the metallic interconnect lines as well as the spaces between them must decrease. Because of current carrying requirements, the thickness of the interconnect lines cannot be reduced. In the fabrication of modern semiconductor devices it is more important than ever to be able to anisotropically etch aluminum and aluminum alloys.
Wet chemical etching processes for aluminum films are not viable processes for modern semiconductor devices because the isotropic nature of wet etching results in a large loss of cross-sectional area. Wet etching processes are generally inadequate for defining features less than 3 .mu.m. Sputter etching with an inert gas is a directional process, but because the selectivity between the material to be etched and the underlying material is usually poor, this technique has found only limited applications. Reactive ion etching processes have been thought to best meet the requirements of VLSI technology. These processes are often directional and can have good selectivity. Reactive ion etching of aluminum and aluminum alloys in gas mixtures containing a chlorinated reactant have been used successfully to form interconnect wirings for VLSI circuits.
Various reactive ion etching techniques for aluminum are taught in an article by Geraldine C. Schwartz entitled "Reactive Plasma Assisted Etching of Aluminum and Aluminum Alloys", recorded in the proceedings of the fifth symposium on plasma processing. One such method utilizes CF.sub.4, BCl.sub.3 and Cl.sub.2 as the gases for the reactive ion etch. The gases are present at a ratio of Cl.sub.2 :CF.sub.4 .gtoreq.1:1 and BCl.sub.3 :Cl.sub.2 .gtoreq.2:1. Chlorine is the species that reacts with and removes the aluminum. The carbon containing gas (CF.sub.4) coats the aluminum side walls as they are exposed and protects them from the reactive species. The role of BCl.sub.3 has historically been debated. It has been thought that because Cl.sub.2 reacts and CF.sub.4 protects, if the ratio of CF.sub.4 :Cl.sub.2 is increased to greater than unity then the etch rate of the aluminum would decrease and eventually go to zero.
The major disadvantage with etching aluminum by the described prior art method is the photoresist undercutting and the lack of anisotropic etching. The prior art method yields an etch bias of between 0.25-0.3 .mu.m. That is, the measured photoresist line width is 0.25-0.3 .mu.m larger than the resulting aluminum line width. This is a very large difference and one which is inadequate in the manufacturing of modern devices where the conducting lines can be less than a micron and separated by only about 0.8 .mu.m. In these cases, the resulting lines are at least 30% narrower than desired, presenting performance and reliability problems for the created integrated circuits. Such narrow lines increase the potential for electromigration in the integrated circuit. Such narrow lines also cause poor metal overlap of contacts, and poor surround of vias. The alignment tolerance of later process steps can also be reduced because of the poor metalization. Another disadvantage with the prior art method is the inability to accurately and consistently determine the end point of the aluminum etch.
It is appreciated that what is needed is a method to anisotropically etch an aluminum metal layer in a conventional reactive ion etcher without undercutting a photoresist mask and to provide a close process control over the etching cycle.